The long-range goal of our proposed work is to help obtain a realistic view of cytoplasmic organization within CNS neurons and to determine how this fundamental cellular organization contributes to important facets of neuronal development and function. The proposed strategy involves use of the high voltage electron microscope (HVEM) to study whole mount preparations of cultured CNS neurons. This essentially novel approach affords a view of neurite and growth cone structure that is remarkable in its resolution and three-dimensional perspective. High resolution ultrastructure will be correlated with developmental and functional states of the neurons. The proposed work will be done with cultured neurons from embryonic chicken retina. This system, with its abundance of local circuit interactions, is well-suited to culture and widely used in developmental studies. Seven experiments are proposed which focus on the following three aims: l: To identify and to sequence structural events that occur as embryonic CNS neurons differentiate in culture. 2: To begin elucidating the functional significance of major identified structures vis-a-vis control of neurite extension, synapse formation, and directed movement of molecules and membranes. 3: To begin determining, at a molecular level, the composition of the observed ultrastructural network. These new aims are logical extensions of initial work done during the past l8 months. Successful completion of these aims should provide novel and fundamental insight into structure-function relationships in developing CNS neurons.